Image coding method and device on basis of wide-angle intra prediction and transform

Abstract

An image decoding method according to the present document comprises the steps of: obtaining intra prediction mode information and an LFNST index; deriving an intra prediction mode of a current block on the basis of intra prediction mode information; when wide-angle intra prediction is applied to the current block, deriving a modified intra prediction mode by means of intra prediction mode remapping; deciding an LFNST set comprising LFNST matrices on the basis of the modified intra prediction mode; selecting one of the LFNST matrices on the basis of the LFNST set and LFNST index; and deriving transform coefficients for the current block on the basis of the selected LFNST matrix.

What is claimed is: 
     
       1. An image decoding method performed by a decoding apparatus, comprising:
 obtaining intra prediction mode information and a low-frequency non-separable transform (LFNST) index from a bitstream; 
 deriving an intra prediction mode for a current block based on the intra prediction mode information; 
 derives prediction samples for the current block based on the intra prediction mode; 
 performing an LFNST to derive transform coefficients; and 
 deriving residual samples for the current block based on the transform coefficients, 
 wherein performing the LFNST comprising: 
 performing a wide-angle intra prediction mode mapping process based on a width of the current block being not equal to a height of the current block; 
 determining an LFNST set among four LFNST sets based on a modified intra prediction mode by the wide-angle intra prediction mode mapping process, the LFNST set comprising LFNST matrices; 
 selecting one of the LFNST matrices based on the determined LFNST set and the LFNST index; and 
 performing a matrix operation based on the one of the LFNST matrices, 
 wherein the intra prediction mode information indicates any one of a planar mode (0), a DC mode (1), and directional modes (2 to 66), 
 wherein the modified intra prediction mode is derived as any one intra prediction mode in a range from −1 to −14 based on the height of the current block being greater than the width of the current block, and 
 wherein an LFNST set for intra prediction modes in a range from −1 to −14 is a same as an LFNST set for intra prediction modes in a range from 2 to 12. 
 
     
     
       2. The image decoding method of  claim 1 , wherein the modified intra prediction mode is derived based on which of the height and the width of the current block is greater, a ratio between the height and the width of the current block, and the intra prediction mode for the current block. 
     
     
       3. The image decoding method of  claim 1 , wherein the LFNST index indicates whether the LFNST is applied or one of the LFNST matrices comprised in the LFNST set. 
     
     
       4. An image encoding method performed by an image encoding apparatus, comprising:
 deriving an intra prediction mode for a current block; 
 deriving prediction samples for the current block based on the intra prediction mode; 
 deriving residual samples for the current block based on the prediction samples; and 
 performing a low-frequency non-separable transform (LFNST) to derive transform coefficients, 
 wherein performing the LFNST comprising: 
 performing a wide-angle intra prediction mode mapping process based on a width of the current block being not equal to a height of the current block; 
 determining an LFNST set among four LFNST sets based on a modified intra prediction mode by the wide-angle intra prediction mode mapping process, the LFNST set comprising LFNST matrices; 
 selecting one of the LFNST matrices based on the determined LFNST set; and 
 performing a matrix operation based on the one of the LFNST matrices, 
 wherein the intra prediction mode information indicates any one of a planar mode (0), a DC mode (1), and directional modes (2 to 66), 
 wherein the modified intra prediction mode is derived as any one intra prediction mode in a range from −1 to −14 based on the height of the current block being greater than the width of the current block, and 
 wherein an LFNST set for intra prediction modes in a range from −1 to −14 is a same as an LFNST set for intra prediction modes in a range from 2 to 12. 
 
     
     
       5. A non-transitory computer-readable digital storage medium that stores a bitstream generated by the image encoding method of  claim 4 . 
     
     
       6. A transmitting method of data for an image, comprising:
 obtaining a bitstream for the image, wherein the bitstream is generated based on deriving an intra prediction mode for a current block, deriving prediction samples for the current block based on the intra prediction mode, deriving residual samples for the current block based on the prediction samples, performing a low frequency non-separable transform (LFNST) to derive transform coefficients, and encoding image information related to the transform coefficients; and 
 transmitting the data comprising the bitstream, 
 wherein performing the LFNST comprising: 
 performing a wide-angle intra prediction mode mapping process based on a width of the current block being not equal to a height of the current block; 
 determining an LFNST set among four LFNST sets based on a modified intra prediction mode by the wide-angle intra prediction mode mapping process, the LFNST set comprising LFNST matrices; 
 selecting one of the LFNST matrices based on the determined LFNST set; and 
 performing a matrix operation based on the one of the LFNST matrices, 
 wherein the intra prediction mode information indicates any one of a planar mode (0), a DC mode (1), and directional modes (2 to 66), 
 wherein the modified intra prediction mode is derived as any one intra prediction mode in a range from −1 to −14 based on the height of the current block being greater than the width of the current block, and 
 wherein an LFNST set for intra prediction modes in a range from −1 to −14 is a same as an LFNST set for intra prediction modes in a range from 2 to 12.